Grain oriented electrical steel strips (Fe—Si) are typically industrially produced as strips having a thickness comprised between 0.18 and 0.50 mm and are characterised by magnetic properties variable according to the specific product class. Said classification substantially refers to the specific power losses of the strip subjected to given electromagnetic work conditions (e.g. P50 Hz at 1.7 Tesla, in W/kg), evaluated along a specific reference direction (rolling direction). The main utilisation of said strips is the production of transformer cores. Good magnetic properties (strongly anisotropic) are obtained controlling the final crystalline structure of the strips to obtain all, or almost all, the grains oriented to have their easiest magnetisation direction (the <001> axis) aligned in the most perfect way with the rolling direction. In practice, final products are obtained having the grains mean diameter generally comprised between 1 and 20 mm having an orientation centred around the Goss orientation ({110} <001>). The minor the angular dispersion around the Goss one, the better the product magnetic permeability and hence the lesser the magnetic losses. The final products having low magnetic losses (core loss s) and high permeability have interesting advantages in terms of design, dimensions and yield of the transformers.
The first industrial production of the above materials was described by the U.S. Firm ARMCO at the beginning of the thirties (U.S. Pat. No. 1,956,559). As well known to the experts, many important improvements have been since introduced in the production technology of grain oriented electrical strips, in terms both of magnetic and physical quality of products and of transformation costs and cycles rationalisation. All existing technologies exploit the same metallurgical strategy to obtain a very strong Goss structure in the final products, i.e. the process of oriented secondary recrystallisation guided by uniformly distributed second phases and/or segregating elements. The, non metallic, second phases and the segregating elements play a fundamental role in controlling (slowing down) the movement of grain boundaries during the final annealing which actuates the selective secondary recrystallisation process.
In the original ARMCO technology, utilising MnS as inhibitor of the grain boundaries movement, and in the subsequent technology developed by NSC, in which the inhibitors are mainly aluminium nitrides (AlN+MnS) (EP 8.385, EP 17.830, EP 202.339), a very important binding step common to both production processes is the heating of the continuously cast slabs (ingots, in old times), immediately before the hot rolling, at very high temperatures (around 1400° C.) for a time sufficient to guarantee a complete dissolution of sulphides and/or nitrides coarsely precipitated during the slab cooling after casting, to re-precipitate them in a very fine and uniformly distributed form throughout the metallic matrix of the hot rolled strips. According to said known technique, such a fine re-precipitation can be started and completed, as well as the precipitates dimensions adjusted, during the process, in any case, however, before the cold rolling. The slab heating to said temperatures requires using special furnaces (pushing furnaces, liquid-slag walking-beam furnaces, induction furnaces) due to the ductility at high temperatures of the Fe-3% Si alloys and to formation of liquid slags.
Recently, new casting technologies were developed for the liquid steel, to simplify the production processes to make them more compact and flexible and to reduce costs. An innovative technology advantageously utilised in the production of electrical steels strips for transformers is the “thin slab” casting, consisting in the continuous casting of slabs having the typical thickness of conventional already roughened slabs, apt to a direct hot rolling, through a sequence of slabs continuous casting, treating in continuous tunnel-furnaces to rise/maintain the temperature of slabs, and finishing-rolling down to coiled strip. The problems connected to the utilisation of said technique for grain oriented products mainly consist in the difficulty to maintain and control the high temperatures necessary to keep in solution the elements forming the second phases, which have to be finely precipitated at the beginning of the finishing hot-rolling step, if desired best micro-structural and magnetic characteristics are to be obtained in the end-products.
The casting technique potentially offering the highest rationalisation level of the processes and the higher production flexibility is the one consisting in the direct production of strips from the liquid steel (Strip Casting), totally eliminating the hot rolling step. Strip Casting is well known and is utilised in the production of electrical strips, in general, and more precisely of grain oriented electrical strips.
The inventors believe that, for an industrial product, it is not convenient to adopt the strategy of directly producing the grain growth inhibitors necessary to the control of the oriented secondary recrystallisation by means of precipitation induced by rapid cooling of the cast strip, as proposed in the current scientific literature and patents. This opinion derives by the fact, well known to the experts, the level of necessary inhibition (drag force to the grain boundaries movement) is high and must remain comprised within a restricted field (1800-2500 cm−1; in other words, with an inhibition level too low or too high the quality of the end products is impaired. Moreover, the inhibition have to be very evenly distributed through the metallic matrix, in that the local lack of necessary levels of inhibition produces texture defects which critically impair the quality of the end products. This is particularly true if very high quality products (e.g. having B800>1900 mT) have to be produced.